Internal combustion engines produce exhaust containing by-products that present known health hazards to human beings, animals and plant life. Pollutants include, for example, unburnt hydrocarbons, carbon monoxide (CO), oxides of nitrogen (NOx) and other residual species, e.g., sulfur-containing compounds. Emission of these pollutants is, to a certain extent, controlled by exhaust catalysts. To be suitable for use, e.g., in vehicle applications, such catalysts must meet stringent requirements with respect to activity (light-off), effectiveness (e.g., responsive to fluctuating exhaust conditions), long-term activity, mechanical integrity, as well as cost effectiveness. Unburnt hydrocarbon, CO and NOx oxide pollutants have been successfully treated using the so-called ‘three-way’ catalyst. These noble metal containing catalysts are capable of converting a high percentage of pollutants into the less harmful products of carbon dioxide, water (steam) and nitrogen. For example, DE-05 38 30 318 describes the use of noble metals, typically metals of the platinum group, such as platinum, palladium, rhodium and mixtures thereof, as catalytic metals which can effectively convert unburnt hydrocarbons, CO, and NOx oxide pollutants under varying conditions encountered.
In practice, the noble metal is typically supported on a high surface area inorganic oxide, such as alumina. Nanocrystalline alumina is often used as a catalyst support due to its high specific surface area and good thermal resistance to coarsening and sintering at elevated temperatures. The alumina is typically applied or “washcoated” onto a ceramic or metallic substrate, such as in the form of a honeycomb monolith, a wire mesh, or similar structure. The noble metal may be associated with the alumina before washcoating or, alternatively, the noble metal may be applied to the alumina after the alumina has been washcoated.
Other catalyst systems use mixed metal oxides, often also including alumina, as noble metal supports. For example, EP 2 036 606 and EP 2 036 607 describe inorganic oxides comprising aluminum oxide, a metal oxide that does not form a composite oxide with aluminum oxide and at least one additional element selected from rare earth elements and alkaline earth elements. In addition, U.S. Pat. No. 6,335,305 discloses a catalyst that comprises an inorganic oxide support and a noble metal loaded on the support, wherein the support comprises a porous oxide and a composite oxide of the formula (Al2O3)a(CeO2)b(ZrO2)1-b, wherein a is from 0.4 to 2.5 and b is from 0.2 to 0.7.